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### Topic: How does the diode protect the transistor in this circuit? (Read 3104 times)previous topic - next topic

#### Grumpy_Mike

#30
##### Jan 11, 2018, 10:32 pmLast Edit: Jan 11, 2018, 10:34 pm by Grumpy_Mike
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but I tried to make the idea understandable.
It was the idea that was wrong, so no amount of maths manipulation is going to correct that.

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You claim plasma = short circuit = resistance 0 Ohm.
No, can't you even read either. I said:-
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Once a plasma is established it's impedance goes from being very high to very very low. In fact almost like a short circuit.
See that word "almost" in there? Ever tried googling "almost"? It says:-
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almost

not quite; very nearly.
"he almost knocked Georgina over"
synonyms: nearly, just about, about, more or less, practically, virtually, all but, as good as, next to, close to, near, nigh on, not far from, not far off, to all intents and purposes, approaching, bordering on, verging on, nearing;
From This date booklet from a major manufacturer of gas discharge tubes.
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During the glow region the increase of current  flow will create an avalanche effect in gas ionization that will transition the GDT into a virtually short circuit mode and current (dependent on the impedance of the voltage source) will pass between the two conductors.  The voltage developed across the GDT with a short circuit condition is called the "Arc Voltage" VARC.
See how they use the word virtually which is a synonym of my word almost?

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Your "short circuit" plasma generates no power.
Plasmas do not generate power, they carry the power from the current across the tube. Voltage is not an issue, you can not use the W = IV relationship in any meaningful way and you have NO control over the V figure.

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inductor is NOT capacitor. In fact inductor is mostly presented as "inverse" of capacitor because their behaviour is "opposite". Capacitor tries to keep voltage constant and sources (or sinks) "any" current needed to do so until it runs out of energy. So when you short circuit the inductor it does nothing - the current flows in loop unchanged, inductor "is happy" and generates no voltage so wasting no power.
So look what you said here "when you short circuit the inductor it does nothing - the current flows in loop unchanged"
That is total and absolute rubbish. Both a capacitor and inductor store energy. A capacitor stores it as electrons gathering on the plates and an inductor stores energy as a magnetic field. When you short out an inductor you remove the current that keeps up the magnetic field and it collapses. I think you have a very shaky idea about superconductors. In the real world at real temperatures the wire forming a coil has a resistance. When current goes round this, the current causes heat to be generated as it flows through the coil and this takes energy out of the magnetic field which is busy collapsing anyway. The collapsing field causes a voltage to be generated in the reverse polarity of the voltage that caused the field. This is basic physics.

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Only insults?
You started it.

#### Smajdalf

#31
##### Jan 12, 2018, 07:36 am
I am sorry I got dragged into discussion about "impedance of plasma". I know little about it and a quick Google search did not help me. You may be right on this topic but in either case it is only distracting us from the main topic of this (hijaked - sorry OP) discussion.
I really want to have this settled. PLEASE let's stand back, forget insults and start again from basics. I claim when (ideal) inductor with inductance L is being discharged via "ideal" diode (meaning it has constant forward drop Vf at any forward current) the following holds:
I(t) = I(0) - t.Vf/L
for any t greater than 0 and lesser than T where
T = I(0).L/Vf
meaning
I(T) = 0.
Ofc
I(t) = 0
for each t > T.

You have said those equations are rubbish. Please show your equations and I will either show you where yours defy fundamental laws of physics or we find we are speaking about different scenario.

#### Grumpy_Mike

#32
##### Jan 12, 2018, 05:50 pmLast Edit: Jan 12, 2018, 05:55 pm by Grumpy_Mike
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You may be right on this topic
What do you mean "may be"? Are you implying their is any doubt that Bourns do not know what they are talking about?

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I claim when (ideal) inductor
Stop right their, their is no ideal inductor here so that makes a nonsense of everything that follows.

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the following holds:
I(t) = I(0) - t.Vf/L
Can you say in words what you mean by that, it sounds wrong. The dimensions of that relationship do not add up. Time multiplied by voltage divided by inductance does not have the units of current.

#### ElCaron

#33
##### Jan 12, 2018, 09:28 pmLast Edit: Jan 12, 2018, 09:29 pm by ElCaron
The dimensions add up perfectly (A=V/H*s), of course it makes sense to look at an ideal inductor to discuss a principle and I would like to know your version, too.

#### Smajdalf

#34
##### Jan 13, 2018, 01:52 pm
What do you mean "may be"? Are you implying their is any doubt that Bourns do not know what they are talking about?
No. I am implying maybe you don't know what they are talking about. But first things first - let's settle the inductor issue and then we can talk about this.
Stop right their, their is no ideal inductor here so that makes a nonsense of everything that follows.
Let's follow standard scientific procedure - start with idealized components which are easy to understand. When we both agree on "ideal" behavior we may step further and try to determine how parasitic properties of real parts influence this behavior.
The dimensions of that relationship do not add up. Time multiplied by voltage divided by inductance does not have the units of current.
Inductance is measured in henry (H). Wiki claims
H=V.s/A;
time is measured in seconds (s), voltage in volts (V) and current in ampere (A).
s.V/H = s.V/(V.s/A) = A
which contradicts you. What units "time multiplied by voltage divided by inductance" have if not ampere?

#### MarkT

#35
##### Jan 13, 2018, 04:53 pm
Can you say in words what you mean by that, it sounds wrong. The dimensions of that relationship do not add up. Time multiplied by voltage divided by inductance does not have the units of current.

Since the _definition_ of inductance is given by the differential equation.  Vdt = LdI, clearly you're wrong!

I think you are thinking of inductive reactance, not inductance.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### BabyGeezer

#36
##### Jan 14, 2018, 03:42 am
One could argue that no one ever chooses anything - that everything is pre-determined, and we're just playing out a script. And yet, we still use the word "choose", because it simplifies the discussion. That's what I chose to do in this case - simplify the discussion. But then, I think you knew that.
sorry to diverge from the inductance debate

to bring the water analogy back into it; this is how i understand the "diode protection" happening;
imagine two pipes connected to a water source (5m up from the ground) - one runs off into a basin 4m up from the ground, while another runs off straight onto the ground.
a trickle would probably reach the basin, but most of the water would flow through the pipe down to the ground.

am i correct in understanding this analogy of electricity/water flow ?

#### Smajdalf

#37
##### Jan 14, 2018, 08:43 am
sorry to diverge from the inductance debate

to bring the water analogy back into it; this is how i understand the "diode protection" happening;
imagine two pipes connected to a water source (5m up from the ground) - one runs off into a basin 4m up from the ground, while another runs off straight onto the ground.
a trickle would probably reach the basin, but most of the water would flow through the pipe down to the ground.

am i correct in understanding this analogy of electricity/water flow ?
1) Water source 5m up from ground is a good analogy for charged capacitor or chemical battery. Not for an inductor. Analogy for an inductor is possibly some rotaty pump or peristaltic pump with small friction (= resistance of the inductor) and large mass (= inductance). If the pump has no external power to do the pumping it is analogy for an inductor. If the pump is powered it may be considered as a current source.
2) Magnitude of the water flow depends not only on potential difference but on width of the tubes (= resistance). If you have thick tube to the 4m basin you will get higher current flow than very narrow tube leading into the 0m basin.

#### polymorph

#38
##### Jan 14, 2018, 07:13 pm
Wow... Grumpy Mike, when you are wrong, you really double down.

These examples use zener diodes because the effect of current decay is more pronounced than with a resistor.

Switching Inductive Loads with Safe Demagnetization - Application Note - Maxim

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The diode must be able to handle the initial current at turnoff, which equals the steady-state current flowing through the inductor when the switch is closed. In addition, the voltage rating
for the diode needs to handle the swing between positive- and negative-voltage levels. A rule of thumb is to select a diode rated for at least the amount of current the inductor coil draws
and at least twice the voltage rating of the operating voltage of the load. For many applications, especially those found in industrial applications that have many output channels per IO
card, this diode is often physically quite large and adds significant extra cost to the BOM.

The other major disadvantage of the simple freewheel diode approach is that it lengthens the decay of current through the inductor. As explained in "Coil Suppression Can Reduce Relay
Life," this slow decay of current can create problems such as "sticking" between relay contacts. For applications where the current must decay quicker, an alternative solution is to use a
Zener diode as shown in Figure 4, which gives a faster current ramp rather than an exponential decay. When the switch opens, the current is shunted through the general-purpose diode
and Zener diode path, maintaining a voltage equal to the Zener voltage (plus forward diode drop) until the inductor energy is dissipated.
Coil Suppression Can Reduce Relay Life

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This diode shunt provides maximum protection to the solid state switch,
but may have very adverse effects on the switching capability of the
relay. It is important to realize that the net force available to cause the
armature to open is the difference between the magnetic restraining
forces and the spring opening forces, that each of these is varying in a
manner to cause the net force to vary both with time and armature
position. It is this net force which gives rise to the armature system
velocity and energy of momentum as it attempts to effect armature and
contact spring transfer.

A slowly decaying magnetic flux (the slowest is experienced with a simple
diode shunt across the coil) means the least net force integral available
to accelerate the armature open. In fact, rapid loss of the opening forces
supplied by stiff NO contact springs, coupled with slowly decaying
magnetic forces, can actually cause a period of net force reversal where
the armature velocity is slowed, stopped, or even momentarily
reversed until the flux further decays, finally permitting available spring
"return" forces to cause transfer to continue.

...

The more rapidly the coil current decays, the less the magnetic hold
back, and thus the greater the armature momentum and contact stick
"break-ability."

Obviously, this is optimized when no suppression is used. However,
near optimum decay rate can be obtained by using a Zener diode in
series with a general purpose diode. When the coil source is interrupted,
the coil current is shunted through this series arrangement, maintaining
a voltage equal to the Zener voltage (plus forward diode drop) until the
coil energy is dissipated. This is illustrated in Fig. 3.

The Zener voltage value is chosen to limit the coil switch voltage to a
level acceptable to the switch rating. This affords the best compromise
both to coil switch protection and relay switching performance, and should
be employed to assure maximum relay performance and reliability while
providing protection to the control circuit from coil induced voltages.
Inductive Loads and Diode Protection

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The best solution is to put a diode across the inductor, as shown at left. The diode must be able to handle the initial diode current, which equals the steady current that had been flowing through the inductor; something like a 1N4004 is fine for many cases.

When the switch is on, the diode is back-biased (from the dc drop across the inductor's winding resistance).
At turn-off the diode goes into conduction, putting the switch terminal a diode drop above the positive supply voltage.
The only disadvantage of this protection circuit is that it lengthens the decay of current through the inductor, since the rate of change of inductor current is proportional to the voltage across it.

For applications where the current must decay quickly (high-speed impact printers, high-speed relays, etc.), it may be better to put a resistor across the inductor, choosing its value so that Vsupply + IR is less than the maximum allowed voltage across the switch. For fastest decay with a given maximum voltage, a zener could be used instead, giving a ramp-down of current rather than an exponential decay.

Source: "The Art of Electronics" by Horowitz and Hill, Cambridge University Press, copyright 1980, ISBN 0-521-23151-5
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

#### Smajdalf

#39
##### Jan 15, 2018, 05:49 pm
Grumpy_Mike, you said I am an idiot and what I am telling is rubbish. Yet you didn't manage to bring anything to support your claims so far. I and other forum members brought evidence you are wrong. I expect you either present something to explain yourself or you apologize. Or are you so coward you will ignore this thread and pretend it never happened?

#### Resinator

#40
##### Jan 15, 2018, 07:42 pm
You started it.
You sound like a little kid, you started it lol, actually Mike it was you calling the guy an idiot saying hes wrong and all the stuff you have typed. Oh the irony!

You are often wrong and I am yet to see you ever admit it, its no big deal because no one knows everything and we all even forget what we knew it happens all the time to me. Only someone who is faking it pretends they know everything, science itself is quite limited and it has a long long way to go and it wont ever have all the answers. No one even knows how a single atom works, its a complete WTF to everyone thats ever existed

I used to think all the answers are out there waiting to be discovered and science was going to do it but the more I have learnt about philosophy the more I have come to realise science is so limited and its not just maths that needs to catch up or anything as simple as that, the menu isnt the meal to quote a famous philosopher, science is just fitting patterns that everyone can recognise and atheism is just the other extreme after so many thousand years of organised religion both are as unlikely as each other but anyway I digress

This basic science as you call it Mike well its not so basic is it, polymorph provided some great information but its as if you have your tail between your legs and your not man enough to come back here and say oh yeah I actually learnt something

A capacitor and inductor are often touted as the inverse of each other (the duality principle) and they share similarities but so many people dont realise how to actually construct the proper dual circuits, the switch has to go in parallel to the capacitor for instance and lots of people forget one is voltage source the other is current source

To confirm what polymorph posted the time constant of an RL circuit is L/R hence increase the R and decrease the response time it can seem counter intuitive if you dont know it but its exactly true, typing off the bat about this stuff when you are just pretending to be this well seasoned expert will always expose you as a fake

Would be great to see an apology where its due but I wont hold my breath, keep pretending it didnt happen and in time hopefully everyone can forget and you will be able to go back to being this all singing all dancing expert lol

Fakes are everywhere and it can be surprising how long they get away with it

#### BabyGeezer

#41
##### Jan 16, 2018, 06:31 am
...science is just fitting patterns that everyone can recognise and atheism is just the other extreme after so many thousand years of organised religion both are as unlikely as each other but anyway I digress
and i will too then.
(apologies to OP)

i wouldn't say "just", there - it *IS THE WAY* for humans to understand - and Science will admit when it has understood something the wrong way. (if you are getting at the ultimate Truth - then Science is the path to that destination.)

and you are misunderstanding atheism as an "extreme opposite" of religion - you're probably thinking of anti-theism.

#### ElCaron

#42
##### Jan 16, 2018, 09:07 amLast Edit: Jan 16, 2018, 09:18 am by ElCaron
and you are misunderstanding atheism as an "extreme opposite" of religion - you're probably thinking of anti-theism.
He is thinking of strong/gnostic atheism. I don't know anybody who is a gnostic atheist (aside maybe from some historic philosophers who didn't know anything about epistemology).
Everyone I know is an agnostic atheist*, who just says "I haven't seen any good evidence for god, so I assume there is no god as a basis for my choices." People may sometimes mistake them for strong atheists, because most discussions go between atheism and a specific religion, and while you cannot proof that there isn't any god, there is good evidence to assume that all existing religions are nonsense. And since major religions still affect atheists strongly, it is justified to have a strong opinion against them.
This is in my opinion a good essay why atheism is not a religion.

* I don't recognise "agnostic", which is often used in that sense, as an ideology on its on, it is an epistemological position. It can still be combined with atheism (common) or theism (uncommon, but arguably existing, e.g. because of Pascal's Wager, which is a really stupid idea).
You could even combine anti-theism with agnostic atheism, which essentially means  "I haven't seen any good evidence for god, so I assume there is no god as a basis for my choices. Since there is no evidence for any god, all those people who claim a god and try to impose supernatural ideas and morals on other people are dickheads and should be fought."

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#### Resinator

#43
##### Jan 16, 2018, 09:38 amLast Edit: Jan 16, 2018, 09:57 am by Resinator
I was talking about atheism as the idea that the universe just happened. The Big Bang proposes that everything sprang into existence in an instant (quantum fluctuation) for no reason, its all just dumb energy.

Think about that and try to conceive an idea as unlikely. It is in fact just as unlikely as

And god said let there be light

You see I hate organised religion more than most, since the Age of Enlightenment science has done some impressive things but it's like a put down theory trying to counteract the previous millenniums. It's the opposite extreme. One minute a guy on a cloud was the architect the next everything just happened.

Just take a look at reality and existence, isn't it a bit strange that anything exists? It's enough to make you ask WTF

As soon as you say the god word everyone thinks of a bloke looking down and judging it's a ridiculous proposition. That's God with a big G most people are unaware of the idea of a god with a little g. Pantheism like Buddhism and eastern religions where god is all that exists and everything is a part of god, it is in fact the original message of all religions, Jesus was clearly a pantheist and Islam had the same message originally unfortunately the message has been corrupted to control people.

Check out luminaries of pantheism murial in California, look at the brains that believed in this concept, very interesting indeed

I'm not proposing this is the answer it's just a concept few people are aware of and (IMO) its much more believable than any bible.

I could write so much more about this, Immanuel Kant, Alan Watts the psychedelic experience there's a lot more but I'm on my phone and it's tough

#### BabyGeezer

#44
##### Jan 16, 2018, 09:57 am
I was talking about atheism as the idea that the universe just happened. The Big Bang proposes that everything sprang into existence in an instant (quantum fluctuation) for no reason, its all just dumb energy.
again you mistake the meaning of atheism - it's got nothing to do with belief of how the universe happened.

- Atheism is simply NOT believing there is a god(s).
- NOT : believing there IS NO god.

i'm sure there are many scientists who are atheists and *still* disagree on HOW the universe started.

Think about that and try to conceive an idea as unlikely. It is in fact just as unlikely as

And god said let there be light
NOPE - God "said" (nothing more) - let there be light - it's only written in a book of dubious source.

Quantum fluctuations have proper theories explaining HOW it occurs and mathematical models to show how it does. Until someone proves such math in error - Science will be the arbiter of "what is (most likely) true".

@ElCaron; yes, i guess you are correct - it's more suitable for gnostic atheism than anti-theism; since those who "hate God" must acknowledge Her existence ! (oh, wait - that's a Misotheist !)

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